An image forming apparatus of an electrophotographic system includes a developing roller opposite to a photosensitive drum. An electrostatic latent image is formed on the surface of the photosensitive drum by uniformly electrifying a surface of the photosensitive drum and selectively eliminating the electric charge accumulated on the surface of the photosensitive drum by exposure. A developing bias is applied to the developing roller, and as the electrostatic latent image is opposed to the developing roller inline with rotation of the photosensitive drum, toner is supplied to the electrostatic latent image from the developing roller based on potential differences between the electrostatic latent image and the developing roller. Therefore, a toner image is formed on the surface of the photosensitive drum. The toner image is then transferred from the surface of the photosensitive drum onto a sheet, and fixed on the sheet by heating or compression.
The toner carried on the developing roller deteriorates as toner images are repeatedly formed, and electric charge remains on the surface of the photosensitive drum after the toner images are formed on the sheet. The deterioration in toner and the small amount of charge remaining affect the development bias and accordingly the density of the toner image such that the density of the toner image transferred to a sheet deviates from an appropriate density. Thus, Japanese Patent No. 2532073 describes a related art image forming apparatus, which is configured to carry out a process for correcting the developing bias so that a toner image of an appropriate density is formed whenever an action of forming a toner image is carried out a given number of times.